Electrochemical methods for the generation of hydrogen peroxide have been previously developed, studied, and used. Hydrogen peroxide is currently used in chemical synthesis, treatment of wastewater, or remediation of ground water. With present methods, the hydrogen peroxide is produced by the expensive anthraquinone process and subsequently shipped to the point of use. No known approach permits hydrogen peroxide to be produced on-site without the need for acids and additional materials. Hydrogen peroxide must therefore be stored and shipped to sites for use, causing safety and time problems as well as undue expense.
Hydrogen peroxide is currently produced by reducing anthraquinone to anthrahydroquinone using hydrogen, followed by its air oxidation back to anthraquinone. In this process, the oxygen is reduced to hydrogen peroxide. This approach does not permit hydrogen peroxide to be generated directly into liquid streams because the direct contact between oxygen and anthrahydroquinone phase contaminates the hydrogen peroxide with the quinone and its degradation and oxidation products. Consequently, present methods of hydrogen peroxide production produce bulk quantities of the chemical which are subsequently shipped to and stored by the hydrogen peroxide user. This leads to substantial expenses for transportation and storage. Direct generation in the liquid stream of concern would eliminate the time and expense of storage and transportation, as well as eliminating associated hazards.
Electrochemical methods currently proposed for the generation of hydrogen peroxide include hydrogen/oxygen fuel cells, dual electrolytic membrane cells, and trickle bed reactors. Details of these approaches will become more apparent in the following discussion of prior art.
Previously used approaches have employed H.sup.+ conducting membranes in a fuel cell configuration, in which oxygen is reduced on the cathode side and hydrogen oxidized on the anode side to supply protons. Other approaches have utilized electrolysis of peroxide ions produced in alkaline media into acid for neutralization and recovery of hydrogen peroxide.
In U.S. Pat. No. 4,758,317, to Chiang, a membrane permeable to hydroxyl ions was used to separate the anode and cathode of an electrolytic cell, in which hydrogen peroxide was generated. Oxygen evolved at the anode was used to feed the cathode. This method does not allow for the direct dispensation of hydrogen peroxide into the desired target liquid.
U.S. Pat. No. 4,753,718, also to Chiang, disclosed an electrolytic cell using an aqueous alkaline electrolyte and porous cathode and anode for generating hydrogen peroxide. Like U.S. Pat. No. 4,753,718, that invention requires access of cathode to air or oxygen gas, and alkaline electrolyte, which, in the absence of acidification of the electrolyte, does not efficiently utilize oxygen, since a portion of the oxygen is reduced to peroxide ion (HO.sub.2.sup.-) rather than hydrogen peroxide (H.sub.2 O.sub.2).
U.S. Pat. No. 4,693,794, by Chiang, contained a prescription for an overall process utilizing aqueous alkaline electrolytes in one or several cells. As with U.S. Pat. Nos. 4,758,317 and 4,753,718, alkaline peroxide solutions must be neutralized with acid to collect all generated hydrogen peroxide and H.sub.2 O.sub.2 precursor (HO.sub.2.sup.-). The acids required for neutralization are an additional expense and safety hazard. Additionally, alkaline media tend to absorb atmospheric CO.sub.2 readily, resulting in clogging of electrodes and necessitating replenishment of the electrolyte and CO.sub.2 free storage. Thus, U.S. Pat. Nos. 4,758,317, 4,753,718, and 4,693,794 have rigorous storage requirements.
U.S. Pat. Nos. 4,533,443 and 4,572,774, by Wrighton, et al. disclosed an indirect electrochemical means for generating hydrogen peroxide. The electro-chemical cell was used to reduce quinone anchored to high surface area support particles suspended in the electrolyte. The suspended particles were removed from the cell and reacted with oxygen to produce hydrogen peroxide. The oxidized anchored quinone was subsequently returned to the electrolytic cell for reduction. This approach requires considerable expense because of the cost of derivatized quinone, and its attachment to the support particles. Additionally, its usage in on-demand applications would be difficult because of the need to separate the supported quinone from water or other liquid streams.
U.S. Pat. No. 4,455,203, by Stucki, discloses an electrolytic process using a OH.sup.- or H.sup.+ (proton) conducting solid electrolyte. The cell utilizes water or oxygen containing gas at the cathode to generate hydrogen peroxide. This approach utilizes solids, rather than polymer gels, as electrolyte resulting in the need for operation at above ambient temperatures and the absence of mechanical flexibility. Temperature limitations result in the cell being incapable of direct interfacing to a stream of water or other liquid to which hydrogen peroxide is supplied.
Another example of an aqueous alkaline electrolyte based electrolytic apparatus for generating hydrogen peroxide is that presented in U.S. Pat. No. 4,430,176, by Davison. That invention utilized a porous cathode into which oxygen and water could be circulated. Application of an electric current resulted in the generation of primarily peroxide (HO.sub.2.sup.-) ion in the alkaline medium. Again, acidification of the peroxide ion containing alkaline medium is necessary to recover hydrogen peroxide.
A method of electrolytically producing and neutralizing peroxide ion is described in U.S. Pat. No. 4,384,931, by Jasinski, et al. and U.S. Pat. No. 4,357,217, by Kuehn, et al. In either case, a three compartment electrochemical cell was used. Two of the compartments (center and anode) were separated by a proton conducting membrane and center and cathode compartments by an OH.sup.- conducting membrane. H.sup.+ ions are produced at the anode and migrate across the H.sup.+ permeable membrane to the center chamber containing aqueous alkaline electrolyte and peroxide ions generated at the cathode. The result is that peroxide ion is converted to hydrogen peroxide in the center well within a self-contained apparatus. The disadvantages of such an approach stems from the usual instability of anion conducting membranes and the presence of salts arising from neutralization of alkaline medium with acid.
Another method for preparing peroxide ions (HO.sub.2.sup.-) is found in U.S. Pat. No. 4,350,575, by Porta, et al. This approach utilized circulation of alkaline electrolyte at the cathode to produce hydrogen peroxide ions in a predetermined concentration. This peroxide ion containing catholyte is then discharged from the cell when the desired concentration level is attained. This approach introduces alkaline contaminants into the hydrogen peroxide collected, which prevents hydrogen peroxide from being dispensed directly into the liquid stream without prior purification.
A method using the reaction of metals with nitrogen and the generation of alkali peroxide is described in U.S. Pat. No. 4,254,090, by Radebold. The method generates alkali metal and alkali peroxide and subsequently reacts these species with nitrogen and water to generate hydrazine and hydrogen peroxide, respectively. The disadvantages of this approach in relation to the current invention stems from the high temperatures required and the noxious chemical species (alkali metal, alkali-peroxide, and hydrazine) evolved.
U.S. Pat. No. 4,067,787, to Kastening, et al. disclosed an electrochemical method of hydrogen peroxide manufacture utilizing an organic redox system in alkaline media. This approach is disadvantageous because of the need for acidification of the electrolyte and the presence of the dissolved organic system.
The process described in U.S. Pat. No. 3,884,777, by Harke, et al. simultaneously electrolytically produces hydrogen peroxide, chlorine dioxide, chlorine, alkali metal hydroxide, and hydrogen. Hydrogen peroxide is produced indirectly by action of pyrosulfuric acid on steam produced at the anode. This method is not directly applicable to direct introduction of hydrogen peroxide into liquid streams at the remote sites because of the several steps required and because of the presence of sulfuric acid in the effluent.
U.S. Pat. No. 5,112,702, by Berzins, et al. discloses an electrolytic hydrogen peroxide synthesis approach using an acidic electrolyte. The method produced hydrogen peroxide under conditions of controlled potential to enhance selectivity to H.sub.2 O.sub.2. Additionally, the electrolyte medium contained halide ions, which tended to improve selectivity.
In addition to the previously discussed patent literature, several recent articles describe hydrogen peroxide synthesis in PEM based fuel cells (K. Otsuka and I. Yamanaka, Electrochimica Acta, 35, 1990 (319)) and electrolytic cells (P. Tatapudi and J. M. Fenton, J. Electrochem. Soc., 140, 1993 (L55); P. Tatapudi and J. M. Fenton, J. Electrochem. Soc., 141, 1994 (1174)). The former (fuel cell) approach utilized hydrogen at the anode of the cell to spontaneously reduce oxygen to H.sub.2 O.sub.2 at the cathode. The latter approach was based on proton exchange membranes interfaced to teflon bonded oxygen reduction and ozone evolution catalysts on the cathode and anode sides, respectively. Oxygen was presented as a humidified stream to the cathode and deionized water to the anode, rather than having hydrogen peroxide be generated in a liquid stream or emulsion of gas and liquid. This poses the disadvantage of having to convert the liquid stream into vapor prior to introduction into the cell.
There is thus a need for an apparatus and method which would allow hydrogen peroxide to be continuously produced on-site without the need for acids and additional materials. The hydrogen peroxide should be produced in streams of liquid in a contaminant-free state, thereby eliminating the expense and hazards associated with storage and transportation of hydrogen peroxide.